A process for producing polyisocyanates having a high content of allophanate and isocyanurate groups is disclosed in JP-A 61-151179. The reaction catalyst employed is, for example, zirconium butoxide.
JP-A 61-151179 does not describe polyisocyanates having a high content of iminooxadiazinedione groups in the polyisocyanates. However, such polyisocyanates are advantageous since they exhibit very low viscosities coupled with high isocyanate functionality and are suitable for use in coating systems for example.
The synthesis of asymmetric trimers of HDI by catalytic oligomerisation is well known and can be carried out by use of hydrogen(poly)fluorides (EP-A 0789 299), especially of tetraalkyl ammonium- or tetraalkyl phosphonium-hydrogenpolyfluorides (EP-A 0896 009 and EP-A 0 962 455), as catalysts.
These catalysts are not commercially available and difficult to prepare. Their synthesis requires the use of the poisonous and very corrosive hydrofluoric acid and is merely possible by use of special reactors and under strong protection.
Moreover, in solvent-free form these catalysts are solids and have to be solved in a suitable solvent bevor they can be used in trimerisation reactions of isocyanates. Usually low-molecular mono alcoholes are used as solvent. The dosage of low amounts of highly concentrated solutions of catalysts in technical scale always is difficult. But in case polyfluorides are used as catalysts the amount of the used solvent must be as low as possible, as the selectivity with regard to the formation of asymmetric trimers is reduced very much in the presence of hydroxyl groups, so that the tendency to form iminooxadiazindion groups decreases significantly.